In recent years, rapid development of the technologies for miniaturization of electronics equipment has brought about popularization of various portable electronic instruments. There is also a demand for miniaturization of batteries that are used as electric power sources for these portable electronic instruments, and thus, non-aqueous electrolyte secondary batteries having high energy densities have attracted public attention.
Particularly, there have been attempts to use elements that form alloys with lithium, such as silicon and tin, and substances which have a high lithium occlusion capacity and a high density, such as amorphous chalcogen compounds. Among them, silicon is capable of occluding lithium up to a ratio of 4.4 lithium atoms per one silicon atom, and the negative electrode capacity of silicon per unit mass is about 10 times the negative electrode capacity of graphitic carbon. However, silicon undergoes a large volume change when subjected to lithium insertion and extraction in charge-discharge cycles, and has a problem with the cycle life such as fine size reduction of active material particles.
The inventors of the present disclosure zealously repeated experiments, and as a result, they found that an active material produced by compositizing finely pulverized silicon monoxide and a carbonaceous material and calcining the composite, in which microcrystalline silicon is dispersed in the carbonaceous material in a state of being included or retained in silicon oxide that strongly binds to silicon, may be obtained, and thereby an increase in the capacity and an enhancement of cycle characteristics can be achieved. However, even in the case of an active material such as this, if the active material is subjected to several hundred charge-discharge cycles, the capacity is decreased, and the service life characteristics are not satisfactory for long-term use.
Furthermore, the inventors of the present disclosure conducted a thorough investigation on the cause of the decrease in the charge-discharge efficiency, and they found that the silicon oxide contained in the active material irreversibly reacts with lithium, and this is a main cause of a decrease in the charge-discharge efficiency in the first charge-discharge cycle. However, when silicon monoxide is used as a raw material as discussed above, the raw material is now SiOx (x=1), and there is a problem that the ratio of silicon and silicon oxide cannot be changed significantly.